This application claims priority of Korea patent Application No. 2000-84, filed on Jan. 3, 2000.
(a) Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same. More particularly, the present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same in which the positive active material has an excellent capacity retention capability.
(b) Description of the Related Art
For positive and negative active materials, rechargeable lithium batteries use a material from or into which lithium ions are reversibly intercalated or deintercalated. For an electrolyte, an organic solvent or polymer is used. Rechargeable lithium batteries produce electric energy by electrochemical oxidation and reduction which take place during the intercalation and deintercalation of lithium ions.
For the negative electrode active material in a rechargeable lithium battery, metallic lithium was used in the early period of development. However, the lithium negative electrode becomes degraded due to a reaction with the electrolyte. That is, lithium dissolved in an electrolyte as lithium ions upon discharging is deposited as lithium metal on the negative electrode upon charging. When charge/discharge is repeated, lithium is deposited in the form of dendrites which is more reactive toward the electrolyte due to enhanced surface area and may induces a short circuit between the negative and positive active material. This may induce battery explosion. Such problems have been addressed by replacing lithium metal with carbon-based materials such as amorphous carbon and crystalline carbon.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiNiO2, LiNi1-xCoxO2 (0 less than x less than 1), LiMn2O4, or LiMnO2. LiCoO2 is commercially used in small batteries since it has good electrical conductivity and relatively high cell voltage, but it is rather expensive. LiNiO2 is less expensive and has high specific capacity, but it is relatively difficult to prepare in the desired quality level. Manganese-based materials such as LiMn2O4 or LiMnO2 are the easiest to prepare, are less expensive than the other materials, and have environmentally friendly characteristics. However, manganese-based materials have relatively low specific capacity. Nevertheless, because of the advantages of manganese-based materials as above, this positive active material is most likely to be used in batteries for electric vehicles and other large-scale systems.
For the manganese-based positive active materials, LiMnO2 has higher specific capacity and better capability to retain its capacity at elevated temperatures, e.g. 50 to 60xc2x0 C., than LiMn2O4. However, LiMnO2 has an extremely low initial capacity of about 30-40 mAh/g, even though its capacity increases to 140 mAh/g (0.2C=0.4 mA/cm2) in about 20 charge-discharge cycles. In addition, it also has disadvantages of multiple plateaus during charge and discharge. Thus, its voltage abruptly decreases through multi-step discharges. Consequently, an electronic circuitry to accommodate this multi-step discharge, is needed at additional cost.
In an attempt to remedy such problems, research is being conducted on Li2Mn2-aCraO4. This material has an initial capacity of 100-120 mAh/g and does not experience an abrupt reduction in capacity on cycling. However, Li2Mn2-aCraO4 has a lower capacity retention capability upon charge-discharge cycling at high temperatures than LiMnO2 (J. Electrochem. Soc. 145(3), 851, 1998).
It is an object of the present invention to provide a positive active material for a rechargeable lithium battery having a good capacity retention capability.
It is another object of the present invention to provide a positive active material for a rechargeable lithium battery having an improved initial capacity.
It is still another object of the present invention to provide a method of preparing a positive active material for a rechargeable lithium battery in which the positive active material has the above characteristics.
These and other objects may be achieved by a positive active material for a rechargeable lithium battery comprising a composite metal oxide represented by the formula 1 or a mixture thereof:
LixMn2-a-bCraMbO4+zxe2x80x83xe2x80x83[formula 1]
where xxe2x89xa72; 0.25 less than a less than 2; 0 less than bxe2x89xa60.3; zxe2x89xa70; M is an alkali earth metal, a transition metal or a mixture thereof.
To achieve the above objects, the present invention provides a method of preparing a positive active material for a rechargeable lithium battery comprising a composite metal oxide represented by the formula 1 or a mixture thereof. In this method, a chromium salt, a manganese salt, and a metal salt(s) are dissolved in a solvent to produce a solution and the obtained solution is heated at 400 to 500xc2x0 C. for a first heat-treatment step to produce a chromium manganese metal oxide. Thereafter, the chromium manganese metal oxide is mixed with a lithium salt and the mixture is heated at 600 to 800xc2x0 C. for a second heat-treatment step.